chore(deps): update dependency webpack-merge to v6

Author: renovate[bot]

package-lock.json

@@ -72,7 +72,7 @@
     "scratch-gui": "github:scratchfoundation/scratch-gui#scratch-desktop-v3.30.1",
     "uuid": "8.3.2",
     "webpack": "4.47.0",
-    "webpack-merge": "4.2.2"
+    "webpack-merge": "6.0.1"
   },
   "resolutions": {
     "upath": "1.2.0"